Mezz has three Sigma shafts: Sigma, Sigma Slim, and Sigma Thick (E, Es, Et). You mentioned your Sigma was heavy ---it sounds like you got Sigma Thick.
As far as the shaft interior? It seems like Mezz's intention was to make a low-deflection wood shaft. It cored out a maple shaft and attached a vault-plate-like ferrule. Just as a carbon-fiber tube needs a vault plate --- so, too, does Mezz's cored-wood shaft. Some more whittling was done by making the shaft pro taper. All of this makes for a light---not a heavy--- shaft.
I am guessing the inside of the Sigma and Sigma Light shafts is foam --- just like the carbon fiber shafts. Sigma Thick's inside? Foam and weights (metal rods, plates, pins, weights?).
(By the way, why are Predator shafts shown on the Mezz's website
https://mezzcue.com/en/products_pool/shafts ?)
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